Conventional methods for producing a fluorine-containing olefin involving a 1,1-dihydro-2-fluorovinyl group generally comprise the following steps: the hydroxyl group of a 1,1-dihydro-2,2-difluoro alcohol (referred to as a "fluoroalcohol" hereinafter) is substituted by a halogen atom; and then the resulting halide is dehalogenated with zinc.
A method for halogen substitution of a fluoroalcohol has been reported, e.g., in J. Am. Chem. Soc., vol. 75, p. 5978 (1953), in which the fluoroalcohol is tosylated and then reacted with a halide such as sodium iodide. The reaction scheme is indicated below. ##STR1##
In the above formulae, R.sub.f represents a perfluoroalkyl group or a fluoroalkyl group (same as in the formulae referred hereinafter).
This method is convenient and useful in laboratory scale production, but has numerous disadvantages, such as (i) expensive reagents (such as p-toluene sulfonic acid chloride and sodium iodide) are used, (ii) in the reaction of the above formula (2), the reaction must be conducted at a high temperature using a high boiling point organic solvent (such as diethylene glycol), (iii) a large amount of waste solvents having a high boiling point must be disposed of. Therefore, this method is disadvantageous for production on an industrial scale.
Another method for halogen substitution of a fluoroalcohol has been reported, e.g., in U.S. Pat. No. 3,038,947, in which the fluoroalcohol is reacted with a thionyl halide in the presence of an amido compound as a catalyst. If the amido compound is not used, the fluoroalcohol cannot be reacted with the thionyl halide. The reaction scheme is indicated below. ##STR2##
This method is convenient and useful compared to the aforementioned method, but, in view of industrial production, has various disadvantages, such as the use of a toxic substance (such as thionyl chloride) and the generation of a large amount of an acidic gas.
The dehalogenation reaction of the thus obtained halide can be conducted using zinc. The reaction scheme is indicated below. EQU R.sub.f CF.sub.2 CH.sub.2 I+Zn.fwdarw.R.sub.f CF.dbd.CH.sub.2 +ZnIF (4) EQU R.sub.f CF.sub.2 CH.sub.2 Cl+Zn.fwdarw.R.sub.f CF.dbd.CH.sub.2 +ZnClF (5)
This reaction is relatively easy, but, in view of the industrial production, has various disadvantages, such as (i) the reaction rate is extremely low when a chloride is used, (ii) an organic solvent such as methanol or dimethylformamide is generally used in the reaction, and the waste organic solvent must be disposed of, and (iii) waste of zinc halide containing non-reacted zinc must be disposed of.
As described above, conventional methods for producing a fluorine-containing olefin have numerous disadvantages when they are practiced industrially.